HONOLULU (Reuters) - A pregnant California woman pleaded guilty in federal court on Thursday to a high seas hoax that she had hoped would cut short a vacation cruise so she could return to her boyfriend.

Kelley Marie Ferguson, 20, of Laguna Hills, pleaded guilty to one count of conveying false information about an attempt to kill passengers aboard a cruise ship. In exchange, the government dropped a second, identical count.

The plea was the result of an agreement with the U.S. Attorney in Honolulu.

Ferguson will be allowed to return to her parents' home in Orange County, California, but U.S. Magistrate Judge Kevin Chang ordered she not leave their home and that she also wear a device that tracks her by satellite.

The decision to let her return home was partly based on the fact that Ferguson is pregnant, said Assistant U.S. Attorney Ken Sorenson.

Sentencing is set for Sept. 22. Ferguson could be sentenced to 20 years in prison and fined up to $250,000.

Sorenson said that dropping one of the charges would have "no practical influence on the sentencing."

"The defendant admitted to her conduct," he said. "I think, to her credit, she realized she had committed a serious offense and wanted to atone for it."

Ferguson had been accused of writing threatening notes April 22 and April 23 while aboard Royal Caribbean's Legend of the Seas during a 10-day cruise to Hawaii from Ensenada, Mexico.

The notes said "all Americanos" would die if the cruise ship and 2,400 crew and passengers docked at a U.S. port, hoping it would quickly return to its point of origin.

The ship was diverted to Oahu, Hawaii, and searched for biological, chemical, radiological and explosive materials by a 120 state, federal and military investigators.

While interviewing people on board, Ferguson, who was traveling with her parents and siblings, confessed to writing the notes, saying she hoped the letters would bring an early end to the trip.